The invention relates to an optical arrangement which converts an entering light beam into an exiting light beam having light which is linearly polarized in the entire cross section essentially in radial direction.
It is necessary to provide projection exposure systems with a very high numerical aperture in order to achieve the highest resolutions in microlithography. Light is coupled into the resist layer at very large angles. When this light is coupled in, the following occur: light losses because of reflection at the outer resist boundary layer and deterioration of the resolution because of lateral migration caused by reflections at the two boundary layers of the resist to the wafer and to the air (formation of standing waves).
The degree of fresnel reflection is then dependent upon the angle between the polarization direction and the reflection plane. The reflection vanishes when light having an electrical field oscillating parallel to the incident angle incidents at the brewster angle. This provides for optimal in-coupling into the resist while at the same time providing maximum suppression of the standing waves.
However, disturbances occur for light which is linearly polarized in one direction as described in European patent publications 0,602,923 and 0,608,572. Accordingly, the apparatus disclosed in these publications generate circularly polarized light which is coupled into the resist as the equivalent of unpolarized light. In this way, homogeneity is achieved over the entire image. However, a loss of efficiency is accepted because in each case, the locally perpendicular polarized light component is intensely reflected.
In European patent publication 0,602,923, it is alternatively suggested that linearly polarized light should be orientated in one direction relative to the orientation of a pattern to be imaged as already disclosed in German patent publication 1,572,195. The penetration via a multiple reflection takes place in the longitudinal direction of the structures and not in the direction of the critical resolution. The efficiency of the in-coupling or the reflection at the resist surface is however not homogeneous.
The effect of the polarization on the reflection at the resist layers and the significance of the fresnel coefficients is described in U.S. Pat. No. 4,899,055 directed to a method for measuring thickness of thin films.
U.S. Pat. No. 5,365,371 discloses a projection exposure apparatus for microlithography wherein a radially directed linear polarization of the light is introduced in order to prevent disturbances because of standing waves in the resist when generating images therein. Two different polarization elements are given, namely, a radial polarization filter composed of a positive cone and a negative cone. This filter is utilized in transmission and effects radial polarization for the reflection because of the fresnel equations. However, it is not disclosed how a complete polarization of the transmitted light is achieved. In the description of U.S. Pat. No. 5,365,371 and in claim 3 thereof, it is required in addition that both parts have different refractive indices. The transmitted part must then however be deflected and cannot pass in a straight line.
U.S. Pat. No. 5,436,761 has a disclosure identical to that of U.S. Pat. No. 5,365,371 referred to above and includes a single claim wherein no condition is given for the indices of refraction. Furthermore, in claim 4 of U.S. Pat. No. 5,365,371, a plate having segments of radially orientated polarization filter foils is given as is known from U.S. Pat. No. 4,286,843 (see FIG. 19 and column 9, lines 60 to 68).
Both polarizers are polarization filters, that is, they lead to high light loss and are suitable only for an incoming light beam which is unpolarized or circularly polarized because, otherwise, an intense nonhomogeneity of the intensity would occur over the cross section of the exiting light beam.
In the example of FIG. 1 of U.S. Pat. No. 5,365,371, the deflecting mirror 17 causes a partial polarization and therefore the light beam exiting from the polarizer 21 is nonhomogeneous.
U.S. Pat. No. 5,365,371 discloses that the radial polarizer lies in the pupillary plane of the projection objective. A position of the radial polarizer in the objective is problematical because there, the tightest tolerances for an optimal image quality must be maintained.
It is an object of the invention to provide an optical arrangement which permits a homogeneous coupling of light into optical boundary surfaces with high aperture and with low loss and low scattered light. It is another object of the invention to provide such an arrangement wherein the efficiency and the homogeneity of the exiting light beam are optimized.
Projection exposure apparatus are provided which permit maximum use of the advantages of radial linear polarization with minimum disturbance of the imaging and minimum complexity with respect to assembly.
The invention is directed to an optical arrangement which includes: an optical structure for receiving an entering light beam; the entering light beam having a linear polarization (P) in a predetermined direction; and, the optical structure being adapted to convert the entering light beam into an exiting light beam wherein the direction of the linear polarization (P) is, however, not subtractively selected but is instead rotated essentially over the entire cross section of the exiting light beam.
In this connection, it is noted that normal polarizers effect a selection. Thus, a polarization direction is permitted to pass and the orthogonals are, for example, removed from the light beam by reflection, refraction and absorption. Accordingly, unpolarized light yields a maximum of 50% linear polarized light. When linear polarized light enters a polarizer at an angle to the direction of polarization, the projection of the polarization vector is selected to the polarization direction for through passage and the orthogonals are eliminated. In contrast, in the optical arrangement of the invention, the direction of the linear polarization is actually rotated.
Advantageous embodiments are disclosed which provide different ways of generating the desired polarization distribution. One embodiment includes ring aperture illumination wherein the incident light at low angles (for which low angles the reflectivity is only slightly dependent upon polarization) is suppressed.
Another embodiment is directed to the integration of a radially polarizing optical arrangement into a microlithographic projection exposure system.
In this system, the possibilities of the optics are fully utilized and an improvement in the homogeneity and in the efficiency of coupling light into the resist layer is achieved because the reflection at the resist layer is reduced uniformly. However, uniform reduction is also achieved at all lenses arranged downstream of the polarizing element. For the light incident at large angles (up to the brewster angle), the effect is the greatest especially where the light intensity (peripheral decay) is at the lowest. The disturbances of the resolution because of scattered light, even at the resist wafer boundary layer, are homogenized and reduced.
An arrangement close to start of the beam path is advantageous because the disturbances caused by stress-induced birefringence at all downstream lenses is minimized and made symmetrical.
For this reason, it is also advantageous for polarization filters (in addition to the preferred polarization-rotating elements) when these elements are mounted in the illuminating system.
In another embodiment, the polarization-rotating elements are mounted at any desired location in a projection illuminating system which is characterized by improved homogeneity and a much higher efficiency compared to the state of the art.
In another embodiment, a reduction and homogenization of the scattered light occurs at each lens of the system (even with a low angle of incidence).
On the other hand, asymmetrical optical elements change the state of polarization and can therefore only be arranged downstream when a reflecting layer having phase correction is utilized. This is especially the case for deflecting mirrors such as for shortening the structural length or as provided in catadioptric projection objectives. If a totally-reflecting prism is utilized as a deflecting element, then a precisely adapted phase-retarding plate must be mounted downstream or the totally reflecting boundary layer must be coated with a phase-correcting layer. Polarizing optical elements such as polarization beam splitters and quarter-wave plates are also disturbing.
The invention is also directed to a microlithographic projection exposure system incorporating a radially polarizing optical arrangement. More specifically, the microlithographic projection exposure system of the invention includes: a light source defining an optical axis and transmitting a light beam along the optical axis; an optical structure arranged on the optical axis for receiving the light beam; the entering light beam having a linear polarization (P) in a predetermined direction; and, the optical structure being adapted to convert the entering light beam into an exiting light beam wherein the direction of the linear polarization (P) is rotated essentially over the entire cross section of the exiting light beam.